Superchargers are relatively easy to fit in the grand scheme of things, I wouldn't attempt it but I am not a mechanic and therefore wouldn't even want to take the head of mine off. Don't get them confused with turbochargers. It isn't "a lot of hassle for 70BHP...", 70BHP is a big power increase!
They are generally pretty much straight bolt ons, and because of that the mapping is easy, you'll be able to use a known map for that specific charger on that engine. Turbos tend to be a lot more work as you have a lot more variety and customisation in the final specification of the engine (going low compression, higher capacity injectors, uprated fuel pump, adjusted boost etc, it all depends what results you were after) which then means the mapping has to be completely custom and as a result Turbos can require a lot of rolling road time to get things just right.
I don't have any experience of supercharging a bimmer engine or that particular brand of charger, but fitting a known brand of SC on a VR block was literally bolt on, update the ECU with a known map, and drive away. You can get the work done in 5-6 hours (the best I heard was 4 hours) and most didn't require any rolling road time at all. I have no doubt this would take longer than 6 hours given the increased complexity of the engine but they aren't a lot of work by any means.
If the kit goes for much under £2000 I'd say someone was getting a good deal. Don't forget to budget for brake upgrades too!
Woots, generally I would guess the thinking would be that if someone wants to add to a super/turbo charger then they would have gone for the larger engine in the first place, they also sell a lot of the 3.0s across the BMW range, You can use forced induction on what you like though, just cause you have a 2.0 doesn't mean it can't be breathed on
